Resonant excitonic contributions to nonlinear optical susceptibilities

We have observed resonant exciton contributions to the nonlinear optical susceptibilities of CdS when the fundamental frequency was tuned through exciton resonances. We show that our data are in good agreement with quantum-mechanical calculations while the anharmonic oscillator model gives incorrect resonance denominators. We also show that selection rules for exciton contributions are different depending on whether the fundamental frequency or generated second-harmonic frequency is in resonance with exciton states.